1. Field of the Invention
The present invention relates to an imaging device.
This application claims priority to and the benefit of Japanese Patent Application No. 2012-051481 filed on Mar. 8, 2012, the disclosure of which is incorporated by reference herein.
2. Description of Related Art
A configuration discussed in Japanese Unexamined Patent Application, First Publication No. 2006-340044 has been known as one of imaging devices using an AD conversion method according to a related art. First, a configuration and an operation of an imaging device discussed in Japanese Unexamined Patent Application, First Publication No. 2006-340044 will be described.
FIG. 14 illustrates a schematic configuration of a (C)MOS imaging device using an AD conversion method according to the related art discussed in Japanese Unexamined Patent Application, First Publication No. 2006-340044. An imaging device 1001 includes an imaging unit 1002, a vertical selecting unit 1012, a read current source unit 1005, an analog unit 1006, a column processing unit 1015, a reference signal generating unit 1016, a horizontal selecting unit 1014, an output unit 1017, a changing unit 1018, and a timing control unit 1020.
The timing control unit 1020 controls each component such as the vertical selecting unit 1012, the read current source unit 1005, the analog unit 1006, the column processing unit 1015, the reference signal generating unit 1016, the horizontal selecting unit 1014, and the output unit 1017. The imaging unit 1002 is configured such that unit pixels 1003 each having a photoelectric conversion element are arranged in the form of a matrix, and generates a pixel signal according to the amount of an incident electromagnetic wave and outputs the pixel signal to a vertical signal line 1013 disposed in each column.
The vertical selecting unit 1012 controls a row address or row scanning of the imaging unit 1002 through the row control line 1011 when driving each unit pixel 1003 of the imaging unit 1002. The horizontal selecting unit 1014 controls a column address or column scanning of a column AD converting unit 1030 of the column processing unit 1015. The read current source unit 1005 serves as a current source for reading the pixel signal from the imaging unit 1002 as a voltage signal. The analog unit 1006 executes amplification or the like as necessary.
The column processing unit 1015 includes the changing unit 1018 and the column AD converting unit 1030 disposed in each column of the imaging unit 1002. The changing unit 1018 is turned on (activated) and short-circuited to a voltage source supplying a predetermined voltage. The column AD converting unit 1030 converts an analog signal which is the pixel signal output from each unit pixel 1003 of the imaging unit 1002 in units of columns into digital data, and outputs the digital data. The reference signal generating unit 1016 is configured with, for example, an integrating circuit or a DAC circuit, and generates a reference signal Ramp whose level changes with passage of time in an inclined manner.
Next, a configuration of the column AD converting unit 1030 will be described. FIG. 15 illustrates a configuration of the column processing unit 1015 including the column AD converting unit 1030. Each column AD converting unit 1030 is configured to include a comparing unit 1031 and a measuring unit 1032 and the column AD converting units 1030 have the same configuration.
The comparing unit 1031 is a comparator circuit that includes a generally well-known differential amplifier as a basic configuration. The comparing unit 1031 compares the pixel signal output from the unit pixel 1003 of the imaging unit 1002 with the reference signal Ramp, and outputs a high-level signal, for example, when the reference signal Ramp is larger than the pixel signal but outputs a low-level signal, for example, when the reference signal Ramp is smaller than the pixel signal.
The measuring unit 1032 is configured with an up/down counter circuit, and measures a comparison time representing a time taken until the comparing unit 1031 finishes a comparison operation after the comparing unit 1031 starts the comparison operation. Thus, a measurement value of the comparison time according to a level of the pixel signal is obtained. The horizontal selecting unit 1014 is configured with a shift register, a decoder, or the like, and controls a column address or column scanning of each column AD converting unit 1030 in the column processing unit 1015. As a result, the AD-converted digital data is output to the output unit 1017 in order through the horizontal signal line.
A comparison operation by the comparing unit 1031 starts after voltages of two input terminals of the differential amplifier configuring the comparing unit 1031 are reset (balanced). The changing unit 1018 is disposed in the imaging device 1001 in order to prevent a trouble in which as a slight variation remains between the voltages of the two input terminals of the differential amplifier configuring the comparing unit 1031 after the reset operation, an output of the comparing unit 1031 is not inverted, or an output of the comparing unit 1031 is inverted immediately after the reference signal Ramp is input. The changing unit 1018 is configured with a switching element that supplies a voltage from a power source VDD to the comparing units 1031 of all columns.
Next, an operation of an AD conversion operation will be described. A concrete operation of the unit pixel 1003 will not be described, but a reset level and a signal level as the pixel signal are output from the unit pixel 1003.
First, after the reading of the reset level from the unit pixel 1003 is stabilized, the voltages of the two input terminals of the differential amplifier configuring the comparing unit 1031 are reset (balanced). Subsequently, a predetermined voltage is applied to the input terminal to which the reference signal Ramp is applied, through the changing unit 1018. Thereafter, the comparing unit 1031 compares the reference signal Ramp with the pixel signal using the predetermined voltage as a comparison start voltage. The measuring unit 1032 executes a measurement in a down-count mode, and a measurement value at a point in time at which a comparison ends is used as digital data of the reset level.
Subsequently, when the signal level from the unit pixel 1003 is read, the reset operation in the comparing unit 1031 and the change operation by the changing unit 1018 are not performed. After the reading of the signal level from the unit pixel 1003 is stabilized, the comparing unit 1031 compares the reference signal Ramp with the pixel signal using the predetermined voltage as the comparison start voltage. The measuring unit 1032 executes a measurement in an up-count mode, and a measurement value of the measuring unit 1032 at a point in time at which the comparison ends is used as digital data of a signal component (a signal obtained by subtracting the reset level from the signal level).
As described above, the pixel signal can be AD-converted. Further, even when a slight variation remains between the voltages of the two input terminals of the differential amplifier configuring the comparing unit 1031 after the reset operation, a predetermined voltage is applied to the input terminal to which the reference signal Ramp is applied, through the changing unit 1018. As a result, the voltage of the input terminal to which the reference signal Ramp is applied is higher than the voltage of the input terminal to which the pixel signal is applied. As a result, the output of the comparing unit 1031 can be reliably inverted during the comparison operation.